wusb: fix find_first_zero_bit() return value check
In wusb_cluster_id_get(), if no zero bits exist in wusb_cluster_id_table,
find_first_zero_bit() returns CLUSTER_IDS.
But it is impossible to detect that the bitmap is full because there
is an off-by-one error in the return value check. It will cause
unexpected memory access by setting bit out of wusb_cluster_id_table
bitmap, and caller will get wrong cluster id.